Larry R. Holt

Congenitally missing lateral incisors are a perplexing challenge for restorative dentists. Various studies indicate that as much as 6% of the population will have partial hypodontia. Congenitally missing teeth occur slightly more often in females than males.1,2 Treatment for this condition should provide esthetics and function in the most conservative manner possible (Figure 1 and Figure 2).

Currently, the preferred method for replacement appears to be the implant-retained crown following orthodontic treatment. Long-term reliability studies leave little doubt that implant-retained crowns are the most stable long-term restoration available. With current treatment modalities, the replacement can be very esthetic and durable (Figure 3).3-6

Regardless of the type of replacement therapy, multiple disciplines will most likely be involved in managing the patient with congenitally missing teeth. Multidisciplinary treatment considerations require excellent communication to obtain the result necessary for restorative excellence. A thorough diagnosis and treatment plan must precede orthodontic therapy.7 The orthodontist must understand critical parameters of root alignment and symmetrical distribution of edentulous spaces. Diagnostic wax-ups at the end of orthodontic treatment can be decisive in determining final tooth position, and consultation with the surgeon who will place the implants is critical.7

Circumstances exist that contraindicate the use of implants:

Patient age and lack of physical maturity: Implants should not be placed until definitive proof of growth completion exists. A good guide is consecutive-year cephalometric radiographs revealing no further growth.8

Inadequate root alignment and spacing: Implants cannot be successfully placed if the appropriate dimension of bone (facial-lingual and mesio-distal) is unavailable in which to place the implant.3,4

Patient is unwilling to have orthodontic treatment.

Inadequate bone architecture in conjunction with a patient unwilling to have bone-grafting procedures.

Patient’s emotional maturity.

Financial concerns: Parents could possibly make compromised decisions based on lack of funds.

Older teenagers and young adults typically go through life transitions that keep them away from dental offices. This is particularly important when considering interim treatments that require frequent recall maintenance visits.

Some situations dictate a postponement of definitive treatment. Missing front teeth are usually diagnosed early in a child’s life. While many of the restorative decisions are made for them by parents and their dentist, the emotional maturity of a patient could indicate postponement of definitive treatment. It can be advantageous to allow a patient to make his or her own decision as a responsible adult. Conservative, reversible tooth replacement schemes fulfill immediate esthetic needs while postponing final decisions concerning definitive replacement therapy.

This article seeks to explore the various treatment modalities available. A literature review of each treatment is part of the exploration. Each potential solution is accompanied by its own set of advantages and disadvantages.

ALTERNATIVE TREATMENTS Canine Substitution The first treatment option is to orthodontically eliminate the need for replacement. This method has the distinct advantage of avoiding any restorative treatment. On the surface, this would seem to be the treatment of choice. European studies have shown that patients treated with canine substitution were more satisfied with their treatment than were patients who had prosthesis.9,10 Current American dental expectations are very high; television and magazines have created an ideal that many patients expect to have fulfilled. Canine substitution may or may not satisfy that ideal.

There are two possible issues with canine substitution. One is esthetic and the other functional. The canine tooth is a very different tooth when compared to a lateral incisor. Its mesio-distal width is typically greater; its facial surface is bulbous and not flat; its incisal edge is not incisor-like; and its gingival contour is typically higher than a lateral incisor. The canine has a much larger root and therefore the likelihood of a “canine eminence” can create an unnatural gingival architecture.11 The canine typically has a deeper chroma than lateral incisors.12 Substitution of a large, dark, angular canine into the delicate confines of the lateral location could be esthetically displeasing (Figure 4).

Canines occupy the corner of the dental arch and are considered indispensable by many practitioners for developing appropriate functional occlusion. Electromyographic studies indicate reduced muscle activity when guidance is provided by the canines.13,14 Guidance is transferred to the first premolars without the canine in its normal location. The classic “canine guided” occlusion cannot be created. Robertson and Mohlin concluded in their study that canine substitution “does not impair temporomandibular joint function, and encourages periodontal health in comparison with prosthetic replacements.”9

Many patients with congenitally missing lateral incisors also have undersized canines.15 This can allow substitution more effectively. An undersized canine can be reshaped and whitened, allowing it to look much like a lateral incisor. Bonding procedures or even porcelain veneers can be done to complete the transformation.10 When the canines are undersized, it is this author’s opinion that canine substitution is a viable treatment option (Figure 5).

Removable Appliances The second available option is a removable appliance. The majority of patients with missing teeth will wear some sort of removable appliance during the course of their treatment. The removable partial denture is often used as an interim solution. It can provide adequate esthetics and function; however, it is not a very satisfying long-term solution (Figure 6). Many patients find a temporary partial denture to be bulky and uncomfortable to wear. Hygiene issues are often a problem, resulting in papillary hyperplasia and generalized gingival inflammation.16-18

Replacement teeth can be attached to space maintainers after orthodontic treatment. This can be an excellent treatment to postpone definitive treatment.7 The patient has a satisfactory esthetic solution and the orthodontic treatment is being stabilized (Figure 7). Temporary replacement may be necessary for several years while waiting for a patient to reach physical maturity. Once growth has been completed, then implant therapy can be initiated.

Three-Unit Conventional Bridge The three-unit bridge can provide excellent esthetics and appropriate occlusal guidance (Figure 8 and Figure 9). Bridges can have a long life expectancy if done with meticulous clinical technique. Patient compliance with appropriate diet, home care, and routine dental maintenance is critical.19 Failure mechanisms for crown restorations are (in order of occurrence): decay, endodontic complication, loss of retention, esthetics, periodontal breakdown, abutment tooth fracture, and porcelain fracture.20 During the course of a young patient’s life, a bridge could require replacement three or four times. Each replacement brings with it more loss of tooth structure and could ultimately lead to the loss of one or both abutment teeth. This is certainly another indication to postpone definitive treatment until alternative treatments are explored.

The major issue with fixed bridgework is the preparation of previously untreated abutment teeth. In adolescents the pulps in the teeth are large and there is a higher chance of endodontic complications.21 Current concepts for preservation of tooth structure would contraindicate the preparation of abutment teeth for a fixed partial denture.22 This concept alone contraindicates the fixed three-unit bridge as a legitimate consideration for lateral incisor replacement on adolescent patients.

“Winged” Bridge (Adhesive-Retained FPD) The “Maryland Bridge” ushered in an era of conservative tooth replacement. The concept was to do minimal preparation of abutment teeth and fashion a porcelain-fused-to-metal pontic with a pair of metal “wings” that would be adhesively attached to the abutment teeth on their lingual side. The treatment originally described by Livaditis was rapidly embraced by many dentists.23,24

A number of issues developed as restorations were placed. The lingual metal generally “telegraphed” through translucent enamel, causing a graying of the abutment teeth.25 This esthetic problem was difficult to overcome. In the laboratory the pontic would be a perfect match to adjacent teeth, but in the mouth the abutment teeth would gray out and the pontic no longer matched.

The chief problem with Maryland bridges was loss of adhesion on one or both retainers.20 This unpredictable complication quickly discouraged many dentists from continuing the treatment. Over time, adhesives improved, preparation technique was modified, and metal etching was refined. Some studies suggest a quite satisfactory medium-term result when following a strict set of guidelines.26,27

“No Winged” Bridge (Adhesive-Bonded Pontic) The “Carolina Bridge,” described by Heymann,28 is a beautifully simple, do-no-harm, fixed replacement for missing lateral incisors. This technique uses a laboratory-fabricated, all-ceramic pontic that is bonded to place using adjacent interproximal surfaces for attachment. The esthetic result of this bridge is excellent (Figure 10 and Figure 11).

The primary limitation of this technique is the amount of interproximal tooth structure available on adjacent abutment tooth surfaces. According to Heymann, a minimum of 5 mm of incisogingival height is necessary to provide adequate bonding sufaces.28 This could potentially require a crown-lengthening procedure. Ideally, occlusion should be in the incisal one third of the anterior teeth. This procedure requires meticulous clinical protocol to ensure success.

The Carolina Bridge avoids many of the problems associated with Maryland Bridges. A key element to the desirability of this treatment is that it involves no preparation of adjacent teeth. The best treatment is always the most conservative for the circumstances presented by the patient. In the case of debonding, this bridge is easily repaired. A variation of the Carolina bridge has been used with adhesively attached denture teeth. This variation can provide a very cost-effective medium-term solution.

The primary disadvantage of this treatment is dislodgement of the pontic. Strict adherence to post-treatment instruction is critical. The inconvenience to both patient and dentist could become a factor with less-than-responsible patients.

All-Ceramic Veneer Bridge The veneer bridge is another attempt at conservative tooth preparation to provide a fixed-appliance tooth replacement. The abutment teeth are prepared for veneers. A three-unit bridge is fabricated with an all-ceramic pontic and two veneer retainers.29

This treatment can be extremely esthetic. It allows for corrections of any imperfections of the abutment teeth. Patient satisfaction with the treatment is very high (Figure 12 and Figure 13).

Unfortunately, this treatment can be short-lived. Marginal failure along the facial margins of the veneers occurs relatively early. At 5 years, the success rate is 75%.30 According to Denissen, “success is dependent on rigid patient selection criteria, strength of the major connectors, and precise adjustment of occlusion to prevent overloading.”30 This statement could be applied to all of the conservative treatment modalities discussed in this article.

This treatment fails to meet the criteria of conservation of tooth structure.22 The facial surface of all anterior teeth is permanently altered. Dissatisfaction with longevity could be a major concern for patients and their parents.

All-Ceramic Winged Bridge The “Drake Bridge” uses a computer-aided design/computer-aided manufacturing (CAD/CAM)-developed zirconia framework. This framework is used as the support for an all-ceramic pontic. The zirconia wings are also porcelain coated. A proprietary process is used by Drake Precision Laboratory (Charlotte, NC) to enhance the bond-strength capability of the wings.

Clinical protocol requires preparation on the lingual surface of both abutment teeth. The preparation should cover as much of the lingual surface as possible. Reduction is typically 0.25 mm to 0.5 mm into enamel with a butt-joint cavo-surface wall. A small box form is created halfway through the interproximal contact at a depth of 0.5 mm. The two box forms should be parallel to one another.

The Drake Bridge is adhesively retained to acid-etched enamel with appropriate bonding agents and a held in place by resin-based veneer cement or composite.

This bridge is extremely esthetic. It does not change the appearance of the abutment teeth. All of the preparation is on the lingual surface and on the lingual aspect of the interproximal. Preparation is minimal; however, it is necessary in most cases unless an open bite is presented (Figure 14 through Figure 17).

The prosthesis is made from materials that are very durable. No long-term studies are available on this restoration. It could be assumed that failure mechanisms will be similar to other winged bridges-the assumption being that debonding of one or both retainers would be the most likely mode of failure. Several studies show life expectancies of anywhere from 7 to 15 years for adhesively bonded winged bridges.20,31-33 As with the veneer bridge, careful patient selection criteria should be exercised. Deep bites are a definite contraindication.

DISCUSSION

Success in almost all dental treatment is dependent on case selection. Matching treatment to the needs of the patient is the primary responsibility. Conservation of tooth structure is critical in treating adolescent patients.

Restorative dentists are faced with three basic options for the replacement of congenitally missing maxillary lateral incisors. Canine substitution, tooth-supported restorations, or implant-retained crowns are the treatment options available.8 The most desirable treatment is no restoration (canine substitution).9,10 Alternatives must be considered because that treatment is not always feasible. The implant-retained crown is now the treatment of choice for many patients. It presents the most predictable, long-lasting replacement available.3-6 Implant treatment must be performed following a strict protocol. The patient must be at a level of physical maturity where no further growth will occur. Adequate bone and soft tissue architecture must be available. Implant fixtures must be placed with meticulous surgical precision into a predetermined anatomical location34 determined by the restorative dentist using a diagnostic wax-up. The ideal location is then communicated with a surgical guide.

Removable appliances will always have a place in the lateral incisor replacement protocol. They are not a desirable long-term restoration, but they can serve very well during the process of getting the patient to physical maturity.

The Carolina Bridge is an extremely conservative treatment strategy. It does no harm to adjacent teeth. This restoration provides an adequate medium-term solution. However, the adhesively attached pontics can be relatively easy to debond.

The newest incarnation of the Maryland Bridge is another relatively conservative treatment option. The Drake Bridge provides excellent esthetics, and it uses minimal preparation which is limited to the lingual and proximal surface of adjacent abutment teeth. The framework is strong; excellent bond strengths can be predicted based on modern bonding agents and the excellent bond strength attainable with etched porcelain. It must be mentioned here that no long-term studies exist on the longevity of this bridge. It may be extrapolated that longevity should follow other “winged bridge” scenarios. This being the case, debonding of one or both retainers is the primary concern with this treatment. Any patient receiving this treatment (or any tooth-born replacement) must be educated to the importance of frequent recall appointments.

Adhesively attached winged pontics should be considered as an interim treatment. They can be predictably placed and will enjoy a clinical life expectancy of 10 to 15 years. In order to obtain long life, it can be expected that the restoration will need to be reattached at least once or possibly twice.26 This seems a reasonable compromise to gain a fixed replacement, as this treatment will carry the patient well into adulthood.

Technology continues to advance. All-ceramic materials are stronger, and modern adhesives create satisfactory bond strengths, especially in enamel. This is supported by an observation from the Goodacre study: The life expectancy of adhesively attached wing bridges is similar to that of three-unit fixed bridges.20 This flies in the face of conventional opinion; most practitioners would make the assumption that fixed bridgework is a much more definitive, longer-lasting treatment.

The limitation of these techniques is based on their conservative attachment to the abutment teeth; as clinicians, we will never control the myriad ways patients abuse their dentitions and stress restorations beyond their limits.

CONCLUSION

Complete communication of risks and rewards associated with each treatment option is critical, and patients must be made aware of the various treatments available. Each option should be presented with a complete disclosure of longevity and downside risks.16,35 It is the opinion of the author, after review of current treatment options, that definitive replacement of missing lateral incisors should be postponed as long as feasible, possibly into adulthood. This postponement allows full maturation of the patient both physically and emotionally. At that point, implants can be placed predictably or alternative treatments can be thoughtfully considered by the patient. Any interim treatment should be conservative of tooth structure, and yet esthetic. Strict adherence to postoperative instructions is critical for success of interim treatment.